1. Field of the Invention
The invention relates to laser diodes and somewhat more particularly to multi-layered laser diode structures having a cover layer transparent to laser radiation for total reflection of radiation produced in a laser-active layer positioned below the cover layer and having an additional layer positioned on top of the cover layer for supporting a contact thereon.
2. Prior Art
Laser diodes having a multi-layer structure composed of epitaxially deposited layers positioned on top of one another on a gallium-arsenide semiconductor substrate are known. Such multi-layer structures contain a laser-active layer comprised of a gallium arsenide material in which, during current passage, a repopulation necessary for generation of coherent laser radiation is produced.
Because of the necessarily high current densities required for laser diodes, precautions must be exercised during diode construction to insure that threshold current density for laser operation is low and that a good junction contact is present. Both of these conditions lead to a selection of a multi-layer structure having a laser-active layer comprised of a gallium arsenide material, preferably containing a given aluminum concentration, sandwiched between two other layers composed of gallium-aluminum-arsenide. The aluminum concentration in these gallium-aluminum-arsenide layers is, for example, about 35 mol % whereas the aluminum concentration in the laser-active layer is, for example, about 8 mol %. In this manner, the gallium-aluminum-arsenide layers exhibit a refractive step or jump of n.sub.1 /n.sub.2 .noteq.1 (wherein n.sub.1 is the refractive index of the gallium arsenide material in the laser-active layer and n.sub.2 is the refractive index of the gallium-aluminum-arsenide in the two adjoining layers) sufficient for total reflection.
The thickness of the layers sandwiched about the laser-active layer is on the order of magnitude of 1 to 5 times, typically 3 times, the size of the wavelength of the laser radiation generated in the gallium-aluminum-arsenide material.
The typical construction of a laser diode described above includes a substrate, a first layer positioned on the substrate and comprised of gallium-aluminum-arsenide, containing, for example, about 35 mole % aluminum, a laser-active layer positioned on the first layer and comprised of gallium-aluminum-arsenide, containing, for example, about 8 mol % aluminum and a further layer positioned on the laser-active layer and composed of gallium-aluminum-arsenide, similar in composition to that of the first layer.
Since it is possible to apply contacts having satisfactory conductive properties only with difficulty on a gallium-aluminum-arsenide layer having a relatively high aluminum concentration therein, as on the above described further or cover layer (i.e., positioned on top of the laser-active layer), it is common to position an additional layer on top of such further layer. Such additional layer is composed of a gallium-arsenide material and has a substantially smooth exterior surface on which a contact is applied. The gallium arsenide material typically utilized for such additional layer has a high absorption coefficient for a wavelength range in which the generated radiation lies.
During the manufacture of laser diodes, an additional problem arises of being able to ascertain or monitor, during the manufacturing process, whether a given substrate disc for supporting the earlier described four epitaxially-applied layers has such qualities from piece-to-piece and/or charge-to-charge, particularly a high capability for radiation generation and/or a low degree of radiation-free transmission, that the subsequent manufacturing steps are still economically desirable.